فهرست مطالب اصغر سیاه منصوری

In this paper, the effect of the intermediate principal stress on the failure of different rocks and the accuracy of several 3D criteria in estimating failure of the true tri-axial tests results was investigated. Then, applying linear and nonlinear regression, a new 3D criterion was proposed that the criterion can successfully predict failure behavior of deep rocks with accuracy higher than other existing criteria.Failure criteria are relations between the strength properties of rock and applied stresses and used to describe the behavior and failure of rocks. This function is determined based on analytical and empirical methods. Some researchers neglect the effect of intermediate principal stress σ2 in the failure mechanism of rocks (2D failure criteria). However, there are experimental documents showing that the intermediate principal stress affects the failure behavior for at least some of the rocks (3D failure criteria. The intermediate principal stress has considerable effect on failure mechanism of very deep underground rock masses (such as hydraulic fracturing, wellbore and reservoir stability, underground storage in petroleum engineering). In this paper, the effect of the intermediate principal stress on the failure of five different rock types in 175 true tri-axial tests was studied. Also, the accuracy of several 3D criteria (such as Von Mises, Drucker-Prager and Pan & Hudson) in estimating failure of the tests results was investigated. Applying nonlinear regression, a new 3D criterion was proposed based on the true tri-axial tests results. Maximum, minimum, and intermediate principal stresses, along with uniaxial compressive strength and internal friction angle are used in the new criterion. In order to validation of the proposed criteria for other rocks, results of the true tri-axial tests for 190 specimens of seven different rock types were studied.The results of studies showed that the effect of intermediate principal stress is more significant in competent rocks than weak rocks and Pan & Hudson criterion gives the most accuracy with an MRAE equal to 0.24. Von Mises and Drucker-Prager criteria gave the next best results by MRAE respectively equal to 0.28 and 0.44. Also, RSQ and MRAE of the proposed criterion for all type of rocks are equal to 0.94 and 0.06 respectively that the criterion can successfully predict failure behavior of deep rocks with accuracy higher than other existing criteria.

Using salt caverns is one of the most developed methods for fluid hydrocarbon storage. Certainty of caverns stability and simultaneously having an economical working gas ratio, are important factors that rarely have been considered in Iran. Considering salt rock characteristics and high overburden, the gravitational stress can be assumed dominant. In situ stress acts as an external pressure and causes cavern convergence. Since conventional rock supports e.g. shotcrete, rock bolt, etc. are not applicable in the salt caverns; the cavern stability can only be achieved by an internal pressure caused by the gas inside the cavern. The paper deals with salt cavern stability analysis for gas storage. Specific geometrical and geotechnical properties (with the general properties of salt domes located in the Southwest of Iran) have been modeled by Phase2 and FLAC3D softwares. Geometrical and mechanical modeling of these caverns in finite difference and finite element in desired depth, determined the minimum and maximum gas pressure for cavern stability and working gas pressure was calculated as well. Comparison of these results implies that carrot shape cavern is more economical.

Recent developments in making new devices for geophysical surveys and the low costs of these surveys have made the geophysical methods such as compression sound waves velocity (P) and electrical resistivity (ρ) very common in the estimation of physical and mechanical properties of rocks. Raymer and colleagues proposed two new relationships between porosity and compression waves for low porosities and very high porosities. They used an interpolation method to estimate an average porosity between two limits. Studies have shown a good correlation (usually between 0.7 and 0.8) between the physical properties of rocks and the compression sound wave velocity. Compression sound wave velocity is a measure of intact rock wave velocity. Therefore, ignoring the crack effects adumbrates the results and generalizes it to rock mass. In this study, we made some artificial fractures in concrete samples in order to study the effect of discontinuities on the physical properties (P wave and electrical resistivity) in laboratory. In order to provide concrete samples, both B and C mix designs for modeling the fine and coarse grain were prepared. To simulate the various discontinuities in the samples, materials with different cohesion and negligible density of viscosity were used. Paper and talc were used to simulate high and very low cohesions, respectively and foam (unolit) was used for distant discontinuities. To prepare the standard samples, templates were constructed according to the NQ ISRM Standard. 20 templates (the number of samples, including 10 for each mix design) were considered. After preparing the concrete mix design based on both B and C mix designs, and lubricating the templates, the concrete was poured along with placement of discontinuities and artificial porosity. The natural porosities of the samples were determined according to ISRM (1981). The Pundit Device was used to test the compression wave velocity according to the standard ASTM D2845 (1978). To measure the electrical resistivity of the samples, the standard ASTM D5334-08 (2001) was used. This test is done in the full saturation. Since the porosity between individual samples for each type (fine or coarse grain) is constant, an increase in the total porosity is the result of an increase in the number of artificial joints and pores. In other words, increasing the joints (discontinuities) and artificial pores, causes an increase in the total porosity in the samples and a reduction in the wave velocity. Saturated samples followed the empirical model of Raymer and colleagues very well. In case of the specimens with equal artificial pores and joints, the wave velocity in the saturated state was larger than that in the dry state. Changes in the compression wave velocity with an increase in the number of joints are greater in dry samples than in saturated samples. In other words, by increasing the number of the joints, the saturation effect of the fluid filling the pores becomes more obvious. The best fit of the data on the electrical resistivity and total porosity (or an increase in the ratio of the joint volume to the pore volume) is a logarithmic relationship that has a correlation coefficient at least equal to 0.97. Considering that water does not replace with the artificial discontinuities, electrical resistivity increases with an increase in discontinuities.